An Iranian opposition supporter covers his face with his bloodstained hand during clashes with security forces in Tehran in December 2009. Further protests, even peaceful ones, on the 12 June anniversary in 2010 have been severely discouraged by arrests and brutality. Photograph: Amir Sadeghi/AFP/Getty Images

The contrast could not be more striking. A year ago they rallied in their millions, a display of people power draped in green that stunned the world, rattled Iran's theocratic leadership and promised to jolt the entire region.

But this Saturday, on the first anniversary of the disputed elections that gave rise to the biggest challenge to the Islamic republic's authority in its 30-year history, a repeat of such tumult is hard to imagine. Months of brutal repression that included mass round-ups, a succession of show trials, lengthy prison sentences and grisly executions has emasculated the Green movement. Its leaders, defeated presidential candidates Mir Hossein Mousavi and Mehdi Karroubi, have called for a peaceful rally to mark the anniversary. Only a courageous few appear likely to heed the call and brave arrest, beatings or worse.

"I understand why people are no longer willing to pour on to the streets," said the mother of a female student activist, who did not want to be named for fear of exposing her jailed daughter. "If you do so, you can be sure to face any kind of punishment, either being arrested, raped, killed or anything else. I don't think people will come out in the numbers we saw last year.

"But I don't think the absence of protesters means the opposition movement is defeated. They'll find a time again. It can't continue like this."

Mehrdad Khonsari, senior researcher at the centre for Arab and Iranian studies in London, said: "I think there will be people who will turn up and demonstrate peacefully but many will be afraid of serious repercussions well over and above what they do on that particular day.

"The infrastructure for organising a mass turnout just does not exist within Iran in the current circumstances."

The green tide has been reversed by a crackdown that has seen an estimated 5,000 people arrested since last June and 115 executed this year alone. Opposition groups say at least 80 have died in street clashes and in detention, although the real death toll may be far higher. At least six political detainees are on death row after being convicted of mohareb (waging war against God) for their alleged role in the demonstrations.

Activists capable of organising protests have been detained or intimidated into silence and passivity. Leading reformists – such as Karroubi's former aide, Mohammad Ali Abtahi – have been given lengthy prison sentences after televised show trials before being released on onerous bail conditions. Many activists have fled to neighbouring countries such as Turkey. Turkish refugee officials have acknowledged the arrival of around 4,000 Iranians since last June, though some sources suggest that is an underestimate.

Former detainees complain of brutal and degrading treatment, including alleged rape and sodomy. One man, who fled to Turkey, said he was dumped in the street and left for dead after sexual assault. The award-winning film-maker Jafar Panahi – detained in Evin prison for two months until his release on 25 May – described being made to strip naked and stand outside for an hour and a half in the middle of the night.

Evidence shows there is a massive effort to snuff out an anniversary protest. The internet – which enabled protesters to communicate through outlets like Facebook and Twitter – has been used by the leadership to monitor dissent. Activists describe ever-present surveillance on the streets and in cyberspace. Many report receiving threatening phone calls from security agents warning against 12 June activities. The message has been reinforced by warnings from senior revolutionary guard commanders that any demonstration will receive zero tolerance.Human rights campaigners say the crackdown intensified after bloody clashes that marred last December's Shia Ashura ceremonies and resulted in at least nine deaths.

"Up to December we had several occasions where it was possible for displays of dissent but in the last month especially the regime has mobilised all its resources to prevent any kind of protest," said Hadi Ghaemi, director of the New York-based International Committee for Human Rights in Iran. "The message is that peaceful protest is the same as taking up arms against the state. Preventing people coming on to the streets on 12 June will require them to be very explicit with their armed forces — it will make Tehran look like it is under martial law."

With such an onslaught, it is hardly surprising the movement has wilted. But it may have been further undermined by its confused leadership. Still at liberty despite repeated demands by hardliners for their arrest, Mousavi and Karroubi are powerless to challenge the regime or co-ordinate gatherings because of the detention of key aides.

Karroubi – normally renowned for his upbeat defiance – has admitted that the Green movement is effectively leaderless and lacking organisation. "If this movement has a leader except the people, the authorities will quickly eliminate [him]," he recently told the opposition website, Rah-e Sabz. "When Mr Mousavi and I established a four-member committee to help those hurt in the post-election incidents, three of its members were quickly arrested and imprisoned. So the essential task it was meant to carry out was left undone."

The two reformists have tried to shield themselves by stressing their close connections with the Islamic revolution's spiritual leader, the late Ayatollah Khomeini. Mousavi – who was Iran 's prime minister under Khomeini during the 1980s – told his website, Kalemeh, last week that he could not "hide my attachment to the imam [Khomeini]".

But the tactic has worn thin as Khomeini's successor as supreme leader, Ayatollah Ali Khamenei, has stepped up his drive to paint the reformists as heretics in league with Iran's western enemies.

At last Friday's memorial ceremony to mark the 21st anniversary of Khomeini's death, Khamenei noted archly that some of the ayatollah's closest revolutionary associates had subsequently been executed. "The yardstick for passing judgment in the Islamic establishment is the present situation and the late imam [Khomeini] said so," he said.

The point was driven home by the humiliating treatment meted out to Khomeini's grandson, Hassan Khomeini, who has sided with Mousavi and Karroubi in denouncing last year's election as fraudulent. Trying to deliver the official homage to his grandfather, he was forced to abandon his address as thousands of religious hardliners – with Khamenei and President Mahmoud Ahmadinejad looking on – chanted "death to Mousavi".

The incident illustrated how far the reformist leaders have been banished. It also exposed a potentially disabling paradox; that in invoking Khomeini's memory, Mousavi and Karroubi are swearing an allegiance that their opponents do not recognise and many of their followers do not share.

"They are leading people who are very different from them," said Mehdi Khalaji, senior fellow at the Washington Institute for Near East Policy. "I don't think this new generation believe in the ideals of the revolution or in Ayatollah Khomeini. Mousavi and Karroubi as individuals with two specific sets of ideas are maybe not relevant any more. But what they represent is very important."

Amid growing radicalism, the two men have come under pressure to change tactics and abandon their loyalty to a political system that has justified the killings of dozens of their supporters and the jailing of their aides.

But they remain important because of their potential to build bridges to members of the elite privately unhappy with Khamenei's leadership, argues Professor Ali Ansari, director of the Institute of Iranian studies at St Andrews University.

"It's important for them strategically to maintain their links and say, we are not arguing for a complete overthrow of the system, which in a practical sense a lot of Iranians would be very wary of," said Ansari. "The hardline authorities have tried to eliminate this possibility by declaring Mousavi a mohareb. But ironically, labelling him that way forces him to become more radical, meaning he will become more relevant to many more people."

Robert Tait is a senior correspondent for Radio Free Europe. Additional reporting by Saeed Kamali Dehghan